Lead-free solder

ABSTRACT

Lead-free solder comprising Sn, Zn and 0.001 to 0.005 wt. % Ti. The lead-free solder does not contain toxic lead, and has sufficient bonding strength to oxide materials such as glass and ceramics.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This is a continuation application of PCT/JP00/01422 filed onMar. 9, 2000, and a continuation-in-part application of U.S. Ser. No.09/703,882 filed on Nov. 2, 2000.

FIELD OF THE INVENTION

[0002] The present invention relates to lead-free solder for solderingan oxide material, such as ceramics and glass at a low temperature.

BACKGROUND OF THE INVENTION

[0003] Electroplating or electroless plating such as gold plating,copper plating, and nickel plating is conducted on an oxide materialincluding ceramics and glass before soldering thereof. However, anotherpre-plating method preceding soldering is desired because the aboveplating is expensive and complicated.

[0004] Solder of Pb—Sn which can be soldered directly to glass andceramics is disclosed in Japanese patents S49-22299B and S52-21980B.

[0005] However, lead is toxic on the health, the environment, and theecosystem, so lead-free solder is desired.

[0006] The solder of Pb—Sn—Cd—Sb disclosed in the above Japanese patent49-22299B is possible to be soldered directly to an oxide material suchas glass and ceramics. However, toxic lead eluates from the solder inquantity to cause serious problem, to the environment, when the soldercontacts acid rain.

[0007] The solder disclosed in the above Japanese patent S52-21980Bcontains rare earth materials which are useful for joining oxidematerials such as glass and ceramics. However, the solder has the sameproblems as above, because it comprises lead as a main component.

[0008] Lead-free solder has been investigated for mounting electronicparts. For example, solder of Sn—Ag—In is disclosed in Japanese patentH9-326554A, and solder of Sn—Zn—Bi system is disclosed in Japanesepatent H8-164495A. However, their bonding strength are not enough tometal oxide materials such as glass and ceramics.

[0009] Solder of Sn—Ag—Al—Zn for soldering metal oxides is disclosed inJapanese patent S55-36032B. This solder easily separates from oxidematerial such as glass and ceramics, because the coefficient of thermalexpansion of the solder is greatly different from that of the oxidematerial.

[0010] The above solders can be used as a sealing material which seals aperiphery of a double-glazing unit comprising two glass plates arrangedparallel with a space therebetween. When manufacturing thedouble-glazing unit, the solder is introduced from a solder feedingapparatus having a solder tank and a feeding line to the space betweenthe glass plates via an introducing plate which is inserted into thespace.

[0011] However, when the conventional solder is fed into the solder tankand kept for a predetermined time, a (Sn, Zn) Ti series compound isdeposited on a bottom of the solder tank, whereby the compound blocksthe feeding line of the solder feeding apparatus and the solder can notbe fed stably.

DISCLOSURE OF THE INVENTION

[0012] It is an object of the present invention to solve problems ofprior art mentioned above and to provide lead-free solder comprising notoxic lead, being capable of bonding strongly to oxide materials such asglass and ceramics and preventing producing the above (Sn, Zn), Tiseries compound so that the solder can be fed stably.

[0013] The solder of the present invention comprises Sn and Zn as maincomponents and further comprises at least one of Ti, Al and Cu.

[0014] It should be noted that a content value of each componentrepresents a mean composition in the solder because Zn, Ti and Al in thesolder are very easily oxidized, and tend to segregate at the surface ofthe solder.

[0015] A first aspect of the lead-free solder of the present inventioncomprises Sn, Zn and 0.001 to 0.005 wt. % Ti.

[0016] The lead-free solder of the present invention may furthercomprise 0.001 to 0.005 wt. % Al.

[0017] The lead-free solder of the present invention may furthercomprise 5.0 to 9.0 wt. % Cu.

[0018] The lead-free solder of the present invention preferablycomprises Sn and Zn such that a ratio of Sn to Zn (Sn/Zn) is 4.0 to19.0. Where each of “Sn” and “Zn” represents a weight percentage thereofin the solder. In case that the lead-free solder contains 91 wt. % Snand 9 wt. % Zn for example, the ratio of Sn/Zn is 10.1 (91/9).

[0019] The lead-free solder of the present invention comprises Sn and Znmore preferably such that the ratio of Sn to Zn (Sn/Zn) is 4≦Sn/Zn<9 and12<Sn/Zn≦19, and contains substantially no Cu. “Substantially no Cu”means that Cu content is not greater than Cu content included asimpurity in usual raw material of the solder including raw metals of Snand Zn.

[0020] The lead-free solder of the present invention may comprise one ormore than two elements among Bi, Si and Sb in the range of 3.2 to 10 wt.%.

[0021] The lead-free solder of the present invention may comprise 0.001to 1.0 wt. % Si.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a graph showing a relationship between a temperature atwhich the solder is kept and an amount of a deposited compoundconcerning two solders having a different Ti content from each other;and,

[0023]FIG. 2 is a graph showing a relationship between a Ti content andan amount of deposited compound when a solder is kept at 270° C.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0024] The composition of the lead-free solder of the present inventionis as follows. The content of components will be represented with apercentage by weight.

[0025] Sn (tin) is not toxic and gives good wetting property againstmaterials to be joined, and Sn is an indispensable constituent for thesolder. Zn (Zinc) is comprised in the solder in order to improveadhesion strength thereof to oxide materials such as glass and ceramics.

[0026] Ti (titanium) is extremely easy to be oxidized, but it bonds thesolder to an oxide material strongly. When Ti content is less than 0.001wt. %, the solder does not bond to an oxide material firmly enough. WhenTi content is more than 0.005%, heat cycle resistance decreases due toincrease of hardness of the solder, and further the solder becomes toohard to use due to a rise of a melting point thereof. Accordingly, Ticontent is preferably 0.001 to 0.005%.

[0027] Al (aluminum) is also extremely easy to be oxidized, and Al makesthe solder to bond to an oxide material firmly. When Al content is lessthan 0.001 wt. %, the solder does not bond the oxide firmly. When Alcontent is more than 0.005%, heat cycle resistance of the solderdecreases due to increase of hardness of the solder, the solder has anincreased melting point, and the solder loses workability. Accordingly,Al content is preferably 0.001 to 0.005%.

[0028] Cu (copper) has a good effect on mechanical strength of thesolder. This effect is insufficient when Cu is less than 5.0%. When Cuis more than 9.0%, the melting point of the solder increases, themechanical strength decreases and a lot of Cu—Sn intermetalic compoundsgrow. Accordingly, Cu content is preferably 5.0 to 9.0%, more preferably0.01 to 3.0%.

[0029] In the lead-free solder of the present invention, the ratio of Snto Zn (Sn/Zn), where Sn and Zn are expressed with a percentage byweight, is preferably 4.0 to 19.0. Sn and Zn make eutectic reaction inan alloy, and eutectic structures therein consist of a minute mixture offine Sn phases and Zn phases. The eutectic structures have highflexibility, so that stress applied to the solder is easily dispersed inthe eutectic phases. The stress does not concentrate on the interface ofthe solder and oxide material such as glass, so that the solder hardlyseparates from the oxide material.

[0030] When the lead-free solder of the present invention has the Sn/Znratio between 9.0 and 12.0 where Sn and Zn are percentages thereof byweight, the solder preferably contains substantially no copper. When thesolder has the Sn/Zn ratio of 9.0 to 12.0, the composition of the solderis near or equal to the ratio of the eutectic composition (Sn/Zn=10.1),and proeutectoids of Sn or Zn hardly grow large so that the eutecticstructure of the solder becomes fine. A solder containing a lot ofeutectic structures has high flexibility so that it is suitable forsoldering glass and the like as mentioned above. However, grains of theeutectic phases grow large when the solder contains a lot of seedcompounds. Cu and Zn in the solder tend to react with each other to formCuZn intermetalic compounds. The compounds work as seeds for of theproeutectoids, so that the eutectic grains grow large. When the grainsgrow large in the solder, the stress applied thereto concentrates on thegrain boundary and causes fracture. Accordingly the solder is preferableto contain substantially no Cu.

[0031] The lead-free solder of the present invention may contain one ormore elements among Bi, Si and Sb in a range 3.2 to 10%. Bi and Siimprove wettability of the solder. Sb improves an appearance of thesoldered solder and increases creep resistance of the solder. The soldermay contain further another element such as Cr, Be, Fe, Ni, Co, Ga, Geand P in a small amount in order to improve wettability and mechanicalstrength of the solder.

[0032] When Si is less than 0.001%, above effects are achievedinsufficiently. Si of more than 1.0% raises the melting point of thesolder so that workability of soldering is lowered. Accordingly Sicontent is preferably 0.001 to 1.0%, more preferably 0.01 to 0.1%.

[0033] The lead-free solder of the present invention may contain In(indium). The In decreases the melting point of the solder, improveswetting property of the solder, and improves flexibility of the solder,so that the stress applied to the interface of the soldered solder andoxide material is relaxed.

[0034] The lead-free solder of the present invention directly solders tonot only an oxide such as glass and ceramics but also metal such as Al,Ti, and Zr which is hard to be soldered due to a metal oxide filmthereon.

[0035] It is preferable to use an ultrasonic equipment which givesultrasonic vibration to the solder during soldering in case ofhardsoldering materials. It is also preferable to use an equipmenthaving a member which transmits a physical stimulus to the interface ofthe solder and the hardsoldering material to promote bonding to eachother. The member may have a shape of a plate or a rod. The member maybe rotated or vibrated.

[0036] Hereinafter, the present invention will be described referring toexamples.

EXAMPLES 1 To 24

[0037] A soda-lime glass plate (50×50×3 mm) was used as a material to beadhered with lead-free solder. The lead-free solder has a compositionshown in Tables 1, 2 and 3. The solder was soldered to the glass usingan ultrasonic soldering iron having a tip which vibrates at 60 kHz. Thecompositions in the tables are represented with a percentage by weight.

[0038] Adhering property of the solder to the glass was estimated byknifing the solder on the glass with a knife. In Tables 1, 2 and 3, acircle mark (◯) of the adhering property shows that more than half ofthe solder remains on the glass, and a cross mark (X) shows that thesolder peels off in its entirety. TABLE 1 Compositions of the solder (wt%) Example 1 2 3 4 5 6 7 8 9 10 Sn 90.85 89.8 91.5 91.2 92.5 88.6 91.292.2 86.5 93.2 Zn 8.99 9.9 8.35 7.6 7.1 11.1 8.39 7.41 12.4 6.2 Ti 0.160.2 0.15 1 0.4 0.3 0.08 0.05 0.1 0.15 Al 0 0.1 0 0.2 0 0 0 0 0 0.1 Cu 00 0 0 0 0 0.33 0.34 1 0.35 Sum 100 100 100 100 100 100 100 100 100 100adhering ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ property

[0039] TABLE 2 Compositions of the solder (wt %) Example 11 12 13 14 1516 17 18 19 20 Sn 90.5 80 89.9 95 80.7 70 50 40 90.05 60 Zn 9.0 19.859.0 3.7 9.8 29.98 49.99 56.5 9.9 39.99 Ti 0.15 0.03 0.5 1 2 0.007 0.0051.5 0.05 0.003 Al 0 0.07 0.5 0.1 2.5 0.005 0.002 1.5 0 0.003 Cu 0.350.05 0.1 0.2 5 0.008 0.003 0.5 0 0.004 Sum 100 100 100 100 100 100 100100 100 100 adhering ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ property

[0040] TABLE 3 Compositions of the solder (wt %) Example ComparativeExample 21 22 23 24 1 2 3 Sn 75 65 87 45 93.7 89 70 Zn 24.8 32 10.5 49.50 0 0 Ti 0.07 0.3 0.7 2.5 3.5 0 0 Al 0.05 0.7 0.3 2 2.8 1 30 Cu 0.08 21.5 1 0 10 0 Sum 100 100 100 100 100 100 100 adhering ◯ ◯ ◯ ◯ X X Xproperty

[0041] As shown from Tables 1, 2 and 3, each of the solder of thepresent invention adheres to the glass firmly, since it contains atleast one of Ti, Al, Cu, Sn and Zn. The solder solders glasses eachother firmly, since it has high mechanical strength, and relaxes thestress applied to the interface between the glass and the solder duringthe solder is cooled. The solder of the present invention does not peeloff when impact is applied thereto after it is soldered.

COMPARATIVE EXAMPLES 1-3

[0042] Table 3 shows compositions and adhesive properties of comparativeexamples. The compositions are represented with a percentage by weight.

[0043] In the comparative examples 1 to 3, contents of Zn and Ti are outof the scope of the present invention. In the comparative example 2, acontent of Cu is out of the scope of the present invention. In thecomparative example 3, a content of Al is out of the scope of thepresent invention. The adhesive property between the lead-free solder ofthe comparative examples and the glass is inferior, so that all thesolder separates or peels off completely from the glass.

EXAMPLES 25-34

[0044] A soda-lime glass plate (50×50×3 mm) was used as a material to besoldered. Lead-free solder shown in Table 4 was used. The solder wassoldered to the glass plate using the ultrasonic soldering iron havingthe tip which vibrates at 60 kHz. Compositions shown in Table 4 arerepresented with a percentage by weight.

[0045] The adhesive property between the glass and the lead-free solderwas estimated by knifing the solder adhered on the glass in the same wayas in the Examples 1-24. In the adhesive property shown in Table 4, acircle mark (◯) shows that more than half of the solder does notseparate but remains on the glass, and a cross mark (X) shows that thesolder separates from the glass in its entirety. TABLE 4 Compositions ofthe solder (wt %) Example 25 26 27 28 29 30 31 32 33 34 Sn 83 84 86.876.8 81.3 90.4 89.99 90.5 90.45 90.49 Zn 9 8.5 8.7 7.7 8.1 9.0 9.0 8.59.0 9.0 Ti 0.15 0.05 0.15 0.15 0.15 0.08 0.105 0.15 0.15 0.159 Al 0 0 00 0 0 0 0 0 0 Cu 0.35 0.35 0.10 0.30 0.35 0.35 0.35 0.35 0.35 0.35 Sb 32 1 5 0 0 0 0 0 0 Si 0.5 0.1 0.2 0 0.05 0.02 0.005 0 0 0 Bi 1 3 2 5 0 00 0 0 0 In 3 2 1 5 10 0.1 0.5 0 0 0 Cr 0 0 0.05 0 0 0 0 0 0 0 Be 0 0 00.05 0 0 0 0 0 0 Fe 0 0 0 0 0.05 0 0 0 0 0 Ni 0 0 0 0 0 0.05 0 0 0 0 Co0 0 0 0 0 0 0.05 0 0 0 Ga 0 0 0 0 0 0 0 0.5 0 0 Ge 0 0 0 0 0 0 0 0 0.050 P 0 0 0 0 0 0 0 0 0 0.001 Sum 100 100 100 100 100 100 100 100 100 100adhering ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ property

[0046] As shown in Table 4, each solder of the examples contains thecomponents of the invention and components such as Cr, Be, Fe, Ni, Co,Ga, Ge, and P. The solder solders glasses each other firmly, since ithas high mechanical strength, and relaxes the stress applied to theinterface between the glass and the solder during the solder is cooled.The solder does not peel off when impact is applied thereto after it issoldered.

[0047] The inventors have found that a Ti content of the solder has aninfluence on the deposition of the (Sn, Zn) Ti series compound depositedon the bottom of the solder tank.

[0048] The inventors conducted experiments for the Ti content whichprevents deposition of the compound and enables to feed the solderstably.

[0049] The inventors, at first, fed a Ti containing solder having acomposition of (Sn—Zn eutectic with Ti) into a solder tank, and meltedthe solder at a certain temperature to obtain a melt of the solder. Themelt was kept at a predetermined temperature for 10 minutes, and thenthe melt was flown out of the tank and the deposition on the bottom ofthe tank was recovered to measure the weight thereof. An amount of thedeposition (%) was calculated according to the following equation:

[amount of the deposited compound (%)]

=[amount of the recovered compound (g)/amount of the melt flown out of

the tank (g)]×100.

[0050] The results were shown in FIG. 1 wherein two kinds of soldershaving different Ti contents were described.

[0051]FIG. 1 shows that the amount of the deposited compound becomesless as the Ti content decreases.

[0052] The inventors, referring above, conducted experiments for a Ticontent of the solder which can be fed stably at 270° C. It should benoted that a solder is usually fed at 270° C. at a usual plant.

[0053] The results were shown in FIG. 2 where a relationship between theTi content of the solder and the amount of the compound depositedtherefrom at 270° C., at which the solder was kept.

[0054] As shown from FIG. 2, the solder having the Ti content of equalto or less than 0.007 wt. % preferably equal to or less than 0.005 wt. %has the deposition amount of the compound of not more than 2.4%. Thesolder of the deposition amount of the compound of not more than 2.4%can be fed stably without a practical problem at a usual plant.

[0055] Industrial Capability

[0056] As mentioned above, the lead-free solder of the present inventiondoes not contain toxic lead, and contains components according to theinvention, and a small amount of Cr, Be, Fe, Ni, Co, Ga, Ge and P. Thesolder solders glasses each other firmly, since it has high mechanicalstrength, and relaxes the stress applied to the interface between theglass and the solder during the solder is cooled. The solder does notpeel off when impact is applied thereto after it is soldered.

What is claimed is:
 1. Lead-free solder comprising Sn, Zn and 0.001 to0.005 wt. % of Ti.
 2. Lead-free solder as claimed in claim 1, wherein aratio of Sn to Zn (Sn/Zn) is 4.0 to 19.0.
 3. Lead-free solder as claimedin claim 1, wherein the ratio of Sn to Zn (Sn/Zn) is 9.0 to 12.0, andsaid solder contains substantially no Cu.
 4. Lead-free solder consistingessentially of Sn, Zn, 0.001 to 0.005 wt. % of Ti, and 0.001 to 0.005wt. % of Al, wherein a ratio of Sn to Zn (Sn/Zn) is 4.0 to 19.0. 5.Lead-free solder consisting essentially of Sn, Zn, 0.001 to 0.005 wt. %of Ti, and 0.001 to 9.0 wt. % of Cu wherein a ratio of Sn to Zn (Sn/Zn)is 4≦Sn/Zn<9 and 12<Sn/Zn≦19.
 6. Lead-free solder consisting essentiallyof Sn, Zn, 0.001 to 0.005 wt. % of Ti, 0.001 to 0.005 wt. % of Al, and5.0 to 9.0 wt. % of Cu, wherein a ratio of Sn to Zn (Sn/Zn) is 4≦Sn/Zn<9and 12<Sn/Zn≦19.
 7. Lead-free solder consisting essentially of Sn, Zn,0.001 to 0.005 wt. % of Ti, and at least one element selected from agroup consisting of Bi, Si and Sb in a range of 3.2 to 10 wt. %, whereina ratio of Sn to Zn (Sn/Zn) is 4.0 to 19.0.